1. Field of the Invention
This invention relates generally to the structure and method of assembling a compact electronic device. More particularly, this invention relates to a new configuration and assembling process for manufacturing compact electronic assemblies with reduced lead-wire connections and solder joints to achieve improved operational power supply efficiency with better thermal performance and meanwhile achieving improved assembly reliability.
2. Description of the Prior Art
As many discrete components are interconnected by lead wires, which are soldered at solder joints to form the power supply systems, the power utilization efficiency and packing density are degraded when the number of solder joints are increased. In addition to the problems caused by interconnecting these discrete components by lead wires, the packaging configuration for each of these discrete components also involves the use of lead frame and wire bonding, which also requires solder joints. The difficulties of efficiency degradation and greater volume assemblies occupying larger space are compounded because of these packaging and assembling configurations.
Referring to FIG. 1A for a circuit diagram of a forward converter commonly employed in a switching power supply (SPS). An induced current is generated and inputted to a inductor winding 12 from a secondary winding 101 via a transformer 10. A set of rectifiers 111 and 112 then rectify the induced current to generate the output DC current to an output circuit (not shown) connected between a positive terminal (Vo) and a negative terminal (-Vo). The rectifiers 111 and 112 can either be a diode or a metal-oxide-semi-conductor field effect transistor (MOSFET). FIG. 1B is a circuit diagram of an alternate forward converter arranged slightly differently than that of FIG. 1A.
Referring to FIG. 2 for a perspective view of a packed MOSFET 111 according to conventional packaging process. A MOSFET die 21 is connected by bonding wires 241 and 242 to a source lead 231 and a gate lead 233 respectively provided on a lead frame. The MOSFET power transistor 21 is supported on a copper tap 22. A drain of the power MOSFET is formed on the bottom surface of the substrate of the die 21 and is connected via the copper tab 22 to the lead 232. The MOSFET rectifier 111 is enclosed and protected in an epoxy molding 25. This packaged MOSFET rectifier 111 is then employed as a component and further assembled as part of the forward converter of FIGS. 1A or 1B, according to FIG. 3 as explained below.
FIG. 3 is a cross sectional view of an electronic assembly connected between a positive terminal +Vo and a negative terminal -Vo for implementing a circuit as that shown in FIG. 1B. One end of the secondary winding 101 is connected to the Copper trace 311 on the printed circuit board through solder joint 321. The source lead 231 of the MOSFET 111 is connected to the Copper trace 311 through solder joint 322. Wire bonding 241 is exploited to connect source lead 231 and MOSFET die 242. The MOSFET die 21 is attached to the copper tab 22 through joints 323 and the copper tab 22 is supported and attached to a second copper trace 312 on the printed circuit board via joints 324. One end of inductor winding is connected to the copper trace 312 through solder joint 325. The other end of inductor winding 12 is connected to the positive terminal +Vo. According to the configuration of assembly shown above, there are multiple joints, i.e., solder joints 321, 322, 323, 324, and 325 composed of Sn62%/Pb36%Ag2%. For a power supply, these multiple joints electrically function as resistors and power losses at these joints significantly reduce the operational efficiency of the power supply systems. For a forward converter with regular size, an assembly as that shown in FIG. 3 results in a total resistance of approximately 875 micro-ohms. Since the power loss is proportional to the resistance and the square of current, i.e., I2R, for a power supply operated with higher current, the power loss becomes a significant design problem that cannot be overcome because of the configuration of assembly that involves so many interconnecting solder joints.
In addition to the problem of the power loss, the packing density of the power supply system is also adversely affected. The packing density is a function of the size of the components and the distance between the components. In a switching power supply system of high power density, there are multiple rectifiers arranged in parallel to increase the efficiency. The connecting traces connected between these joints thus occupy large percentage of the packaging space. As a conventional forward converter requires so many solder joints, the packing density cannot be easily reduced.
Therefore, an improved packaging and assembling configuration is still required to simplify the structure and interconnections of these discrete components. This simplified structure must be able to reduce the number of joints while providing stable and reliable interconnections. It is further desirable that the improved configuration can be carried out in mass production processes that can be conveniently automated to further reduce the production cost of the assembled systems.